Cleaning and shaping root canals of a tooth to prepare it for receiving filling substances is accomplished with the use of drilling instruments having one active portion, called the working portion, the purpose of which is to form, cut and clean the interior walls of the root canal to prepare it for receiving treatment materials and filling material in order to prevent any contact with oxygen that might cause the development of bacteria within the tooth.
Most instruments for drilling root canals comprise an active portion called the cutting portion with a conical envelope and one or more cutting edges wound in a spiral along the active portion.
The spiral shape of the instrument is crucial for evacuating dental debris towards the outside of the root canal. During use, the conical portion of the instrument, which is the working portion, may undergo a phenomenon whereby the material removed from the canal along the spiral becomes wrapped around the instrument, forming a covering that renders the instrument completely useless. In addition, when this instrument is introduced into the canal and rotated using the hand piece equipped with an electric motor, it may become embedded inside the canal, which leads to blockage and may cause it to break.
For this reason it is recommended that this type of instrument be used only with a back-and forth motion, possibly rotating it slightly and alternating in one direction and the opposite direction, to prevent it from becoming stuck and incurring the distinct risk of breakage, which can be catastrophic due to the difficulties of extracting a fragment of a broken instrument.
Several instruments have been developed to overcome these difficulties. For example, one notable endodontic instrument is cited in Swiss Patent 692 484, which is designed specifically to eliminate, at least partially, the embedding effect by varying the spiral angle of the flutes on the active portion, called the working area of the instrument. Another instrument described in French Patent No. 01 02452 also at least partially prevents this embedding effect due to undulation of the flutes on the active portion of the instrument.
Japanese Publication JP 59 019120 describes an instrument that comprises a working area having adjacent straight segments that alternate with twisted segments. However, these segments are uniform in length, and the flutes do not penetrate inside the conical envelope. Furthermore, the transverse sections of the instrument in the working area do not comprise variations that are useful for both the functions of “cutting the material in the canal wall” and “evacuating the cut material along this wall.”
These two instruments are effective to a certain extent insofar as the desired non-embedding is concerned. However, none of these known instruments can guarantee that embedding absolutely will not occur during use by a practitioner.
Another quality that a good endodontic instrument must possess is cutting efficiency. This property allows dental work to take place without excessive heating or smoothing of the canal walls and thus plugging the tubules of the root canal with infected debris. Efficient cutting also means the instrument works with fewer torsion constraints, which reduces the risk of instrument breakage during use. Moreover, efficient cutting allows the operator to proceed with a lower working torque, leading to increased patient safety if there is breakage during torsion. Finally, efficient cutting allows the root to be treated more quickly, eliminating excessive cyclic fatigue as the instrument follows the root canal curves during functioning.
Conversely, cutting capacity that is too great may lead to a high risk of embedding, which is obviously an undesirable effect and imperative to avoid because of the risk of breakage. Because of this, it is essential when shaping the active portion, called the working area, to strike a compromise between efficient cutting and efficient anti-embedding operations.